1. Field of the Invention
This invention pertains to an apparatus for rapidly starting and stopping a conical scan antenna beam.
2. Description of the Prior Art
All tracking radars use some arrangement to offset the antenna feed to sense angle line-of-sight to the target with respect to the antenna tracking line-of-sight (i.e. the boresight or centerline axis). The feed offset arrangement may be a conical scan, various forms of lobe switching or monopulse. Conical scanning is often preferred in air-to-ground millimeter wave seekers, because it offers less complexity and lower cost than lobe switching or monopulse.
One type of conical scan tracking system used in air-to-ground millimeter wave seekers operates with the conical scan (tracking mode) inactive during the search mode. When a target is detected, the conical scan (tracking mode) is activated to track the target. This system suffers from frequent loss of target as the system switches from the search mode to the conical scan (tracking mode). Loss of target may occur during the several hundred milliseconds required for the motor driving the conical scan feed to reach the speed necessary to generate tracking error signals.
An alternate design eliminates switchover from search to tracking mode by keeping the conical scan activated during the search mode. This design yields a continuous composite (search and tracking) pattern. Thus, when a target is detected there is no mechanical delay in switching from target search to target track. However, the composite scan design requires a complex target detection technique to demodulate the composite scan and provide two-dimensional spatial matched filtering. Such a composite scan system is described in U.S. Pat. No. 4,150,379 issued Apr. 17, 1979 to J. Connors and assigned to the asignee of the present invention.
The present invention avoids the potential loss of target by switching from the search mode to the conical scan (tracking mode) far more rapidly than the previous design. Thus, targets are not lost during the time conical scan is being activated. Further, since there is no composite (search plus tracking) scan during the search mode, a less complex spatial match filter target detection algorithm may be used. The present invention is also less susceptible to jamming since there is no composite scan during the search mode.